Angular accelerometer



4 Sheets-Sheet l IN V EN TOR.

C. yK. STEDMAN ANGULAR ACCELEROMETER Aug. 9, 1960 Filed April 8, 1957Aug. 9, 1960 c. K. sTEDMAN ANGULAR-AccELERoMETER 4 Sheets-Sheet 2 FiledApril 8, 1957 Aug. 9, 1960 C. K. STEDMAN 2,948,153

ANGULAR-ACCELEROMETER Filed April 8, 1957 4 Sheets-Sheet 3 'MIM \h`INVENTOR,

Aug. 9, 1960 c. K. sTEDMAN 2,948,153

ANGULAR ACCELEROMETE R Filed April 8, 195'. 4 Sheets-Sheet 4 CEC/L Z.S750/MiN BY )/wf AN GULAR ACCELEROMETER Cecil K. Stedman, Enumclaw,Wash., assgnor to Statham Laboratories, Inc., Los Angeles, Calif., acorporation of California Filed Apr. 8, 1957, Ser. No. 651,335

8Claims. (CLIS-516') This invention relates to a motion responsivedevice for indicating and recording the magnitude and nature of motionsof an object in space. It belongs to the general class of motion sensingdevices such as accelerom-A eters, in which the elective mass is aliquid. Such devices are described in the Statham Patents Nos.2,760,038, l

2,760,039 and 2,760,040.

In such structures, the motion of the liquid is damped by viscous drag.In order to increase the damping ef fect, there is added an additionaldamping means, by causing the uid to flow through an oriiice. The'result of this arrangement not only gives anadditional damping electbut has the property that it tends to maintain a more nearly constantdamping coefficient for the instrument, notwithstanding temperaturechanges and consequent changes in viscosity of the liquid, than would beobtained if the orifice were not employed.

In the form of my invention, as will be more fully described below, asin the above Statham patents the liquid mass serves as the rotor so thatthe weight of the paddle which must be supported along the sensitiveaxis can be made extremely low. Bales are xedly mounted above and belowthe paddle, such baflies being disposed parallel to the axis of thepaddle' and spaced close' ly adjacent thereto, thus forming twosub-chambers. The

separation between the edges of the paddle and the inner chamber wallsform uid communication passageways, and the spaces between the bales andpaddls form other fluid communicating passages. When the instrument issubjected `to an angular acceleration, the inertial `forces will causethe liquid to circulate and develop a pressure against the paddle. Thepaddlewill deect angularly until the elastic restraint of the flexureand the pickoi (mechanism for sensing the motion) balances the forces onthe paddle. 'Ihe novel structure of the invention can be readily adaptedto the use of a variety of pickotl mechanisms, to record the angulardisplacement of the paddle relative to the case. p

The damping characteristics of such instruments are such that as theviscosity of the liquid decreases, the damping ratio decreases, passesthrough a minimum, and then again increases, as the viscosity decreases.This minimum usually extends over a range of viscosity wherein the value`of the damping ratio changes but in an unappreciable amount. Forconvenience, this damping ratio may beY termed the minimum dampingratio, and the viscosity, or viscosity at the minimum value of thedamping ratio, the minimum viscosity.

For ,any given instrument designt the value of the minimumdampingratiodepends on the'resistance to iiow of the liquid in theorilices and also on resistance to ow in the subchambers, and thefollowing ratio:

x/T R +1 (Eq` 1) where r is the resistance to dow through the paddlegaps and R is theresistance to ow in the subchambers.

is approximately equal to Ice 2,948,153

Patented Aug. 9, 1960 In any given instrument of the above design, lledwith any given liquid, the value of the minimum damping ratio and thetemperature at which this minimum occurs will depend, all other thingsbeing held constant, on the resistance to ow in the gaps, the dampingratio increasing with decrease in this resistance. It has been 'observedthat all other design parameters being constant, a widening of the gapincreases the value of the minimum damping ratio, While Aon the otherhand, a narrowing of the gap results in a reduction in the value of theminimum damping ratio and a reduction in the damping ratio at alltemperatures.

One of the problems associated with the above de,

creases with rising frequency much more rapidly than in an idealinstrument having constant mass, stiifness and damping., In some casesit lirst diminishes and then rises to a resonance peak as the frequencyincreases.v

This phenomenon is termed droop and is an objectionable characteristicfor accelerometers and other types of transducers.` The desirablecharacteristic is to obtain a minimum variation in relative response atfrequencies to at least 0.4 of the natural frequency.

I have found that the droop may be reduced in value and substantiallyeliminated by decreasing the distance between the opposed surfaces ofthe subchambers in which the liquid circulates.

In the preferred embodiment I accomplish this result by introducing intothe chambers, bafdes whose plane extends transversely to the axis ofrotation of the paddles and thus in eiect convert the subchambers into aplurality of parallel elongated ow channels through which the liquidmoves in parallel ow between surfaces which are more narrowly spacedthan the top and bottom of the subchamber in which they are placed. As aresult of this change, not only is the response curve, which shows droopin the absence of the auxiliary bales, much improved without anysubstantial change in natural frequency and damping ratio, but also therange of the instrument is also improved.

i By range is meant the angular acceleration required to produce achosen angular displacement of the paddle. Since in all instrumentsWhose design requirement limits the permissible displacement of thepaddle, usually established by the use of stops, my inventionV increasesthe utility of the device, permitting the device to be employed over awider rangeof accelerations I have found surprisingly that incontradistinction to usual seismic systems, in which a solid mass issuspended on a flexible' `constraint such as a spring, that -I may pro-Educe a large increase in range with but a minor and even unsubstantialincrease in natural frequency and' damping ratio. p

These and other properties of my invention will be further described inconnection with the drawings, of which: v

Fig. 1 is an irregular horizontal section of my device taken on line 1 1of Fig. 2;

Fig. 2 is an irregular horizontal section taken on line 2--2 of FigQ l;i

Fig. 3 is a section taken on line 3 3 of Fig. l;

'Fig 4 is a section similar to Fig. 1 of a modification of thestructure; i

Fig. 5 is a section on line 5--5 of Fig. 4; and

Fig. 6 is a chart showing the improved characteristics of the structureof Fig. 4.

The device shown in the drawings, except for the modification to be morefully described below,y is substantially-identical with the deviced'escribed in my copending application, Serial No. 577,707, except forthe omission of the auxiliary baiie orifices shown in said copendingcase, which may also be used here for like purposes. It is similar tothe device illustrated in Statham patents listed above modified, as willbe more fully described below.

IThe device employs a case 1 in which is carried an expansion chamber Zsealed by a bottom 3 and a cover 4 and separated by a flexible diaphragm5 into an air chamber 6 open to ambient pressure and a chamber 7communicating with the accelerometer chamber 10 via ports 9. Theaccelerometer chamber 10 (see Fig. l) formed by the case 1 is shown ascircular but may be of any desired cross-section. The case is completelyiilled with liquid.

Centrally positioned in the chamber 10 is an island 11 on which ismounted a paddle 12 on a Cardan spring suspension 12', which acts as ayieldable constraint and a hinge for said paddle. The paddle issymmetrical about its center and is statically balanced on the springsuspension. The paddle may be made hollow or otherwise reduced in mass,so as to be buoyant to the degree desired or equal in mass to the volumeof the liquid displaced by the paddle so as to be of substantially noeffective mass. However, if it is desired to introduce linearacceleration sensitivity, the paddle may have a greater mass on one sidethan on the other side of the axis of rotation.

Mounted on the paddle 12 is a soft -iron varmature 13 spacedsymmetrically from the pole pieces 14 and 15 of the lE core 16 of thedifferential electromagnetic transducer on which the electromagneticcoils 17 and 18 are mounted. In this respect the construction is similarto that shown in the Statham Patent No. 2,778,623.

Across the diameter of the chamber 10 are vertically adjustable baiiies19 and 20, mounted so that the desired spacing 21 and 22 may be providedbetween the paddle and baiile. The paddle ends are positioned closelyadjacent to but are spaced from the wall of the chamber.

This provides two subchambers 2X3 and 24. In each of these subchambersare placed stacks of plates 25 and 26 notched out at 27 and 28 toencompass the island 11. The plates are separated by nuts 29 on studs 30and held on the top 3 by means of nuts 31 and are equally spaced fromeach other. This provides a plurality of parallel passageways 32 and 33for circulation of liquid relative to the case on angular displacementof the paddle 12, the said baffles and passageways being xedlypositioned in the case in the preferred embodiment illustrated in thedrawings so that the passageways and the bailles are perpendicular tothe axis of rotation of the paddle on its hinge, the direction of motionof the paddle being in a plane parallel to said passageways and baffles,as is shown in the description in the specification, in this form thedirection of circulation of the liquid in the passageways being alongplates perpendicular to the axis of angular displacement of the paddle.

|`Fig. 4 shows the same construction except that only one plate baffle34 and 35 in each subchamber is mounted on spacers 36 on the island 11so that each subchamber is divided into two parallel circulatorychannels 37 and 38 of equal width. Stop screws 39 are provided to limitthe maximum angular displacement of the paddle so that the gap 40between the armature 13 and the pole pieces 14 and 15 is varied withindesirable limits, as will be understood by those skilled in the art.

The case 1 in the forms shown in Figs. l to 5 is lled bottom of thecase.

with oil, as in the case of the previously mentioned applications forLetters Patent.

The following example illustrates the results obtained by employing myinvention and is not intended to be a limitation of but merelyillustrative of the principles of my invention.

lThe device shown in Figs. l to 3 was tested to determine its naturalfrequency, responsecharacteristics, damping ratio (percent of criticaldamping) and range. This will be referred to as vins'trumer'it #1.The's'ame device, but with the bafiles v32 and 33 removed, was testedunder the same conditions and employing the same The natural frequencyis the frequency at the phase shift point between the paddle and thecase.

The sensitivity is reported as the output voltage in millivolts (rootmean square Voltage) of the bridge used to measure the unbalance of thebridge due to the displacement of the paddle upon application of a givenacceleration. The input voltage to the bridge was l2 volts for bothinstruments. The range is reported as mv (root mean square) volts pervolt input to the bridge per r-adian per sec?. Since the bridgeunbalance is directly proportional to the angular displacement, theratio of the ranges of the instruments l and 2 is equal to the ratio ofthe reciprocal of the sensitivity. That is, the range of instrument 1 asabout twice the range of instrument 2.

The effect of the baffles as shown in Figs. l to 3 is illustrated byFig. 6 in which curve A. shows the respone characteristics of instrument#l and curve B that of instrument #2. On the ordinate is plotted therelative response, i.e., the angular displacement of the paddle atvarious frequencies under constant angular acceleration of the case tothe angular displacement of the paddle at zero frequency (ob-tained byextrapolating to zero frequency the measured outputs of the instrumentat various frequencies under the same acceleration). The abscissa arethe relative frequencies, i.e., the ratio applied frequency (at whichthe above response is obtained) to the natural frequency (i.e., thefrequency at the 90 phase shift p'oint).

yIt will be observed that curve B of instrument '#2 shows bad droopcharacteristics. This -droop has been removed by the introduction of thebafes. The response curve is substantially flat to a frequency of about0.7 of the natural frequency.

While, as I have described above, I prefer to make the devicesymmetrical by providing like baffles, Vpreferably horizontallydisposed, in each subchamber, I may for some purposes omit the bafliesin one subchamber or use unequally spaced or unequally numbered baies inthe subchambers, and may dispose them'at any convenient angle in thesubchambers with their surfaces at any convenient inclination to the topand Instead of employing electromagnetic transducers to sense theangular displacement of said paddle, I may use any other convenientsensing mechanism, and thus I may use the unbounded strain Wire gages asemployed in the structures of the above Statham patents.

` While I have described a particular embodiment of my invention for thepurpose of illustration, it should be understood that variousmodifications and adaptations fthereof may be made within the spirit ofthe invention as set forth in the appended claims.

I claim:

l. A motion sensing device, comprising a case, a liquid in said case, apaddle in said case immersed in said liquid, a mounting for said paddlefor relative motion of said paddle with respect to the case, a yieldableconstraint connecting said paddle to said ease, a baille extending alongsaid paddle and spaced therefrom, dividing said case into a plurality ofsubchambers, the space between the paddle and said bailie forming aliquid communicating passageway from one subchamber to the othersubchamber, and additional batiies in at least one of said subchambers,said additional bales forming with said subchamber a plurality ofelongated passageways in said subchamber between said last named bales,and means for sensing the angular displacement of said paddle withrespect to said case.

2. In the device of claim 1, in which said additional batlies aredisposed in another of said subchambers, said baiiies providing withsaid case a plurality of elongated parallel liquid passageways in saidsecond subchamber.

3. A motion sensing device, comprising a case, a liquid in said case, apaddle in said case immersed in said liquid, a mounting for said paddlefor relative motion of said p'addle with respect to the case, lthedirection of said movement in a plane, said paddle extending across saidchamber to but out of contact with the Wall of said chamber, a yieldableconstraint connecting said paddle to said case, a bale extending alongsaid paddle and spaced therefrom, dividing said case into a plurality ofsubchambers, the space between the paddle and said baille forming aliquid communicating passage- Way from one subchamber to the othersubch-amber, and a plate in at least one of said subchambers positionedparallel to said plane, said plate forming With said case a plurality ofparallel passageways in said subchamber, and means for sensing theangular displacement of said paddle with respect to said case.

4. In the device of claim 3, a horizontally positioned plate in anotherof said subchambers, said last named plate providing with said case aplurality of parallel liquid passageways in said second subchamber.

5. A motion sensing device comprising a case, a liquid in said case, apaddle in said case, a hinge mounting for said paddle, said paddle beingangularly displaceable about said hinge, a yieldable constraint for saidpaddle on said hinge mounting, a baffle extending along said paddle andspaced therefrom, dividing said case into a plurality of subchambers,4the space between the paddle and said baiiie forming a liquidcommunicating passageway from one subchamber to Ithe other subchamber,and a plurality of plates in said subchamber, said plates beingpositioned perpendicular to 'the axis of said hinge, said pl-atesforming with said case a plurality of parallel passageways in saidsubchamber, and means for sensing the angular displacement of saidpaddle with respect to said case.

6. yIn lthe device of claim 5, a plurality of horizontally positionedplates in another of said subchambers, said last named plates providingwith said case Ia plurality of parallel liquid passageways in saidsecond subchamber.

7. A motion sensing device comprising a case, a liquid in said case, apaddle in said case, a hinge mounting for said paddle, said paddle beingangularly displaceable about said hinge, a yieldable constraint for saidpaddle on said -hinge mounting, said paddle extending across saidchamber to but out of contact with the Wall of said chamber, a baieextending along said paddle and spaced therefrom, dividing said easeinto .a plurality of subchambers, the space between the paddle and saidchamber and said battle forming a liquid communicating passageway fromone subchamber to the other subh chamber, a plurality of baiiies in oneof said subchambers, said baiiies forming with said ease a plurality ofparallel passageways in said subchamber, said passageways beingpositioned to provide planes of circulation for said liquid in saidpassageways which are perpendicular to the axis of said hinge and meansfor sensing the angular displacement of said paddle with respect to saidcase.

8. In the device of claim 7, a horizontally positioned baiiie in anotherof said subchambers, said baiile providing with said case a plurality ofparallel liquid passageways in said second subchamber.

References Cited in the le of this patent UNITED STATES PATENTS

